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I’m often asked if compost can be used as mulch.

To better understand the roles of each in our gardens and landscapes is to realize they share similar properties but ultimately one should not be substituted for the other. Think of mulch as the icing on the cake and compost as the ingredients that make the cake taste so good.

Let me explain why the two play important but separate roles in the garden. Let’s start with a better understanding of what compost is and what it does. Since I like to eat and cook almost as much as I like to garden, that’s why I use the analogy of baking a cake. Lets start with the ingredients. Like sugar, a little goes a long way but it’s oh so important to making the difference between a great dessert and one that might look good but no one wants to eat. Well, compost is the sweetener of garden soil. In practical terms, compost is part of your soil, rather than something that is intended to simply cover it like mulch. No matter what condition you soil is in originally, whether excessively heavy and compact or too loose and sandy, compost always makes it better. When mixed with native soil, it breaks up heavy clay to help it drain better. In sandy soil just the opposite happens. With compost in the mix, water doesn’t drain nearly as fast, giving plant roots a chance to absorb moisture before it’s gone.

Compost is chock full of disease fighting organisms and the essential nutrients your plants need to thrive, without the need to use supplemental fertilizers. If you’re an organic gardener, or simply trying to use fewer chemicals, compost is your answer. In my garden, a half-inch layer worked a few inches into my garden soil twice a year is all I need to keep my plants growing strong all season. A little bit really does go a long way. But in order for it to work its magic, it needs to be where the roots are. If you place compost only on the surface, like mulch, it will help, but to really be effective, it needs to be incorporated where the action is.

In the spirit of our baking analogy, mulch on the other hand is the icing that makes the cake all the better. Just imagine having a choice between a slice of cake without icing and the same cake with icing. Unless you’re counting calories, you’ll go for the frosted cake every time. Well in the garden, you should always add the “frosting”. Although organic mulch, like leaves, shredded bark, or straw, will eventually break down and add important nutrients to your soil, the primary purpose of mulch is for it to cover your soil, to protect it and plant roots growing there from extreme temperatures, retain vital moisture, suppress weeds and soil-born diseases. And like icing, it’s aesthetically pleasing…very pleasing!

In my book, it doesn’t matter what type of mulch you use as long as it’s organic. You’ll never find me using shredded rubber tires for example as a substitute for what nature has to offer. Although it might sound appealing to use something that never breaks down so you don’t have to replace it, that’s exactly why I don’t use artificial mulch. Although it works, anything I put on my soil has to improve what’s in my soil when it eventually breaks down.

So there you have it. With compost, just like you could spread sugar all over the outside of your cake in place of frosting, you really don’t want to do that. It’s better mixed into the batter where every bite counts. And as for mulch, like icing, you want something with a little more bulk that has the staying power to weather whatever comes along. That’s why mulch is typically something that spreads rather easily and stays in place once its down. So for me, the best gardens use compost and mulch. Both are very important but serve different roles. It’s not an either, or…it’s both!